Respiratory Mechanics

Boyle’s Law states that when volume increases, pressure ____

Decreases

Air flows from an area of ____ pressure to ____ pressure

High → low

Airflow stops when the pressure gradient is

Zero

The pressure at the airway opening is called ____ or ____

Pao, Pm

Alveolar pressure is written as ____

Palv

Pleural pressure is written as ____

Ppl

Transairway pressure (PAW) = ____ – ____

Palv – Pao

Transpulmonary pressure (PL) = ____ – ____

Palv – Ppl

PL is responsible for keeping the ____ open

Alveoli

When Palv < Pao, air?

Moves into lungs

Pleural pressure is most negative at the

Base

What causes pleural pressure to be negative?

Opposing recoil of lungs and chest wall

Explain why Ppl is more negative at the apex

Gravity pulls lung tissue downward, stretching the apex more → more negative Ppl

Why do alveoli at the apex fill first, but contribute less to gas exchange?

Apex alveoli already start more inflated (higher PL), so they undergo a smaller volume change → less ventilation

Define lung compliance

A measure of how easily the lungs can stretch; ΔV / ΔP

Normal static lung compliance (adult)?

200 mL/cmH₂O

High compliance = lungs are ____

Easy to stretch

Low compliance = lungs are ____

Stiff

Static compliance is measured when airflow is ____

Zero

Dynamic compliance is measured during ____

Active airflow

On a pressure-volume curve, increased compliance creates a ____ and ____ slope

Steeper; upward shift

Hysteresis shows that lung volumes are ____ during expiration at the same pressure

Higher

In mechanical ventilation, positive pressure makes Palv (higher/lower) than Pao during inspiration?

Higher (opposite of spontaneous breathing)

Define elastance

The tendency of the lungs to recoil back to original shape; inverse of compliance

What provides elastic recoil in the lungs? (3 things)

Elastin, collagen, surface tension

If compliance increases, elastance ____

Decreases

High elastance = lungs are ____ to inflate

Difficult/stiff

Why is passive exhalation normally effortless?

It is driven by elastic recoil of lung tissue (elastance)

Define airway resistance (Raw)

Opposition to airflow caused by friction between air and airway walls

The factor with the greatest influence on resistance is ____

Radius

Turbulent flow is most common in the?

Trachea

Heliox decreases resistance because its density is?

Lower

Laminar flow has a Reynolds number less than ____

2000

Turbulent flow occurs when Re is greater than ____

4000

Transitional flow occurs at ____

Airway branch points

Why does asthma increase airway resistance?

Narrower radius → increases velocity → turbulent flow →  increases friction → increases resistance

How do lung volumes affect resistance?

Lower lung volumes → narrower airways → increase resistance

What causes a pressure-volume curve to shift downward/flatten?

Decreased compliance (fibrosis, ARDS)

What causes an upward/steep shift?

Increased compliance (emphysema)

Time constant = ____ × ____

Resistance × Compliance

One time constant fills ____% of a lung unit

63%

Five time constants fill ____%

99%

A lung with high resistance or high compliance has a ____ time constant

Long

Explain Boyle’s Law and how it drives spontaneous ventilation

Boyle’s Law: P ∝ 1/V (inverse relationship)

Inspiration: thoracic volume ↑ → Palv ↓ below Pao → air flows in

Expiration: thoracic volume ↓ → Palv ↑ above Pao → air flows out

Why is a pressure gradient required for airflow?

Because air always moves from high → low pressure. With no gradient (Palv = Pao), airflow stops completely

Describe what happens to Palv during a full breath cycle

Start inspiration: Palv becomes slightly negative (~ -1 to -2 cmH₂O)

Mid inspiration: stays negative

End inspiration: Palv returns to 0 (flow stops)

Expiration: Palv becomes positive (+1 cmH₂O)

End expiration: back to 0

Compare Palv, Ppl, and Pao during spontaneous inspiration

Pao = 0 (baseline atmospheric)

Ppl becomes more negative

Palv becomes negative relative to Pao

How do these pressures differ in mechanical ventilation?

Positive pressure creates Palv > Pao during inspiration (opposite of spontaneous)

Ppl becomes less negative or even positive

Air is pushed into lungs instead of pulled

Define Transairway Pressure (PAW) and give its function

PAW = Palv – Pao
Function: drives air movement through conducting airways

Define Transpulmonary Pressure (PL) and explain why it is essential

PL = Palv – Ppl
It is the expanding force that keeps alveoli open. Higher PL = larger alveolar volume

If Palv = +2 cmH₂O and Pao = 0 cmH₂O, which direction does air flow and why?

Outward, because Palv > Pao → pressure gradient pushes air out

If Palv = 0 and Ppl becomes more negative, what happens to PL and alveoli?

PL increases → alveoli expand more → air flows into lungs

Why do alveoli at the apex start more inflated than those at the base?

More negative Ppl at apex → higher PL → larger initial volume

Which alveoli contribute more to ventilation: apex or base? Explain

Base alveoli.
They are smaller at rest → undergo larger volume change → greater ventilation

What is the equation for compliance, and what does it represent?

C = ΔV / ΔP
Represents ease of lung expansion

What are the two components of total respiratory compliance?

Lung compliance

Chest wall compliance

Total respiratory system = both combined

What is static compliance and when is it measured?

Measured when there is no airflow (plateau pressure). Reflects lung + chest wall elasticity alone

What is dynamic compliance and what does it reflect?

Measured during airflow. Affected by:

airway resistance

flow rate

lung compliance

Lower dynamic C than static C indicates increased airway resistance

Give examples of diseases with high compliance and explain why

Emphysema → damaged elastin → lungs overexpand easily → poor recoil

Give examples of low compliance conditions.

Fibrosis, ARDS, pneumonia, pulmonary edema
→ stiff lungs → more pressure needed for same volume

What does a steeper slope on the PV curve mean?

High compliance (easy to inflate)

What does a flatter PV slope mean?

Low compliance (stiff lungs)

What causes hysteresis in the PV curve?

Surfactant recruitment

Alveolar opening pressures

Inflation requires more energy than deflation

Define elastance

Tendency of lungs to recoil back after expansion.
E = 1 / C

Why are compliance and elastance inversely related?

High compliance lungs stretch easily → low recoil → low elastance

Low compliance lungs resist stretch → high recoil → high elastance

What contributes to elastic recoil?

Elastin fibers

Collagen networks

Surface tension of alveoli (major contributor)

What happens to work of breathing when elastance increases?

Work of breathing ↑ because more pressure is needed to achieve ventilation

Why do emphysema patients struggle with exhalation?

Loss of elastin → ↓ recoil → alveoli don’t collapse well → air trapping

Why do fibrotic lungs have high elastance?

Abnormal collagen deposition → resist stretch → snap back quickly

Define airway resistance.

Opposition to airflow due to friction between gas and airway walls

What equation estimates airway resistance?

Raw = (Pao – Palv) / Flow

Which factor has the greatest effect on Raw and why?

Radius — because small radius changes cause huge resistance changes (to the 4th power via Poiseuille's Law)

How does turbulent vs laminar flow affect resistance?

Turbulent flow ↑ resistance due to random gas movement and wall friction

What is the Reynolds number used for?

To determine airflow type

In which airways do you typically find NR

Laminar flow? → Bronchioles

Turbulent? → Nose, pharynx, larynx, trachea

Why does Heliox help in obstructive airway diseases?

Lower density → less turbulent flow → reduced resistance → easier airflow

Explain why asthma increases resistance

↓ radius + ↑ mucus + ↑ gas velocity → turbulent flow → high resistance

Define a time constant

TC = Compliance × Resistance
Time needed to fill/empty lung units

What % of alveolar filling happens at
1 TC = %
2 TC = %
3 TC = __%

63%, 86%, 95%

Why do high time constant lung units (e.g., emphysema) fill/empty slowly?

High compliance + high resistance → slow to inflate and deflate → air trapping

Why do low time constant lungs (e.g., ARDS) fill/empty quickly?

Low compliance → stiff → fills fast but with little volume

Air is pushed into the lungs by a ventilator using positive pressure, causing
Palv > Pao during inspiration (opposite of spontaneous breathing)

Mechanical ventilation

The amount a spring stretches is directly proportional to the force applied.
In the lungs: Pressure applied ∝ Volume change, until elastic limits are reached

Hooke’s Law

The speed at which air molecules move through an airway.
Smaller radius → velocity increases → more turbulence → higher resistance

Airflow velocity

A fluid's internal “thickness.”
Higher viscosity → higher resistance.
(But viscosity has less impact than density.)

Viscosity

The primary phospholipid in surfactant responsible for reducing surface tension.
Made and stored in lamellar bodies of Type II cells

DPPC (dipalmitoylphosphatidylcholine)

Water-repelling portion of surfactant molecules.
These face away from the alveolar fluid.

Hydrophobic

Water-attracting portion of surfactant molecules.
These face toward the alveolar fluid.

Hydrophilic

A lipid molecule with a hydrophilic head + hydrophobic tails.
Forms the structure of surfactant and all cell membranes.

Phospholipid

Another name for DPPC, the major surfactant phospholipid.
It increases sharply after 35 weeks gestation → important for newborn lung stability

Lecithin

Partial or complete collapse of alveoli → reduced gas exchange.
Occurs when surface tension is high or surfactant is low

Atelectasis

Hydrophilic surfactant protein.
Function: immune defense + regulates surfactant secretion.

SP-A

Hydrophobic surfactant protein.
Function: essential for spreading surfactant across alveoli (failure = fatal in newborns)

SP-B

Hydrophobic surfactant protein.
Function: helps surfactant insert into the monolayer → stabilizes small alveoli

SP-C

Hydrophilic surfactant protein.
Function: innate immune defense, pathogen recognition

SP-D